This invention relates generally to a printing system in which an inked design engraved on a printing plate is imprinted on an object having an irregular surface by means of a conformable transfer pad, and more particularly to a technique for creating a pre-distorted design on the printing plate which when transferred to the irregular surface imprints an undistorted design thereon.
Conventional printing machines, such as offset, intaglio and lithographic presses, are adapted to imprint or decorate planar or uniformly curved surfaces. In order to decorate or imprint objects having irregular surfaces, such as convex or concave-sided containers, multi-faceted dice or odd-shaped articles having ridged or recessed surfaces, it is now known to use the so-called Tampo-Print system. The Tampo-Print system, which is described in the article entitled "Decorating Breakthrough Copes with `Impossible` Plastics Parts" appearing in the March 1972 edition of Plastics World, involves the use of a debossed metal printing plate and conformable transfer pad.
The design, decoration or printed matter to be impressed on the irregular surface of the object is acid-etched in the metal printing plate. In operation, the etched area is filled with pigment by a moving brush or squeegee which repeatedly carries the pigment from a reservoir. A doctor blade then cleans the surface of the plate to assure the absence of pigment from the non-decorated area, after which the transfer pad is pressed down on the plate to conform to and pick up the pigment from the etched area. Finally, the transfer pad is brought into contact with the object to be printed, all of the pigment being transferred thereto. After transfer, the pad is completely clean and free of residue.
The transfer pad is formed of soft, silicone rubber having a teat-shaped configuration and a smooth surface. When the pad is pressed down on the engraved or etched printing plate, the teat-shaped pad flattens out so that its surface engages the inked design thereon. When, thereafter, the soft pad engages the article to be decorated, the pad then undergoes a more complex distortion to conform its surface to the irregular surface of the article, thereby transferring the inked design carried by the pad to the surface of the article.
The nature of the distortion to which the pad is subjected when it is pressed against the flat printing plate to pick up the inked design is obviously quite different from the distortion it experiences when it is forced to conform to the irregular surface of the article being decorated. The difference which exists between the pick-up and impression modes obviously depends on the particular configuration of the article being printed.
In order to provide reasonably undistorted printing on the surface of the article, the present practice is to create a design format for the printing plate which is predistorted to an extent giving rise to an imprint on the article that is substantially free of distortion.
For example, if one is to print a drawing of a cat on a convex surface, optical means may be used in preparing the photographic master from which the printing plate is etched or engraved to pre-distort the image of the cat so that when this figure is transferred from the plate to the convex surface, it is substantially undistorted. In transferring from a flat to a simple convex surface by means of a teat-shaped transfer pad, one can very nearly predict the degree of pre-distortion which is needed to create an undistorted imprint.
But when the article to be printed has a more complex surface geometry, such as the surface of a twelve-sided die or dodecahedron, it is virtually impossible to predict the amount of predistortion which must be introduced in the printing plate format to insure an undistorted imprint on the die by the transfer pad. Hence the Tampo-Print system, using existing techniques for pre-distorting the design format on the printing plate, does not produce satisfactory designs on highly irregular or complicated surfaces.